Résumé

Permafrost thaw ponds are ubiquitous in the eastern Canadian Arctic, yet little information exists on their potential as sources of methylmercury (MeHg) to freshwaters. They are microbially-active and conducive to methylation of inorganic mercury, and are also affected by Arctic warming. This multi-year study investigates thaw ponds in a discontinuous permafrost region in the Subarctic taiga (Kuujjuarapik-Whapmagoostui, QC) and a continuous permafrost region in the Arctic tundra (Bylot Island, NU). MeHg concentrations in thaw ponds were well above levels measured in most freshwater ecosystems in the Canadian Arctic (> 0.1 ng L⁻¹). On Bylot, ice-wedge trough ponds showed significantly higher MeHg (0.3 - 2.2 ng L⁻¹) than polygonal ponds (0.1 - 0.3 ng L⁻¹) or lakes (< 0.1 ng L⁻¹). High MeHg were measured in the bottom waters of Subarctic thaw ponds near Kuujjuarapik (0.1 - 3.1 ngL⁻¹). High water MeHg concentrations in thaw ponds were strongly correlated with variables associated with high inputs of organic matter (DOC, a₃₂₀, Fe), nutrients (TP, TN), and microbial activity (dissolved CO₂ and CH₄). Thawing permafrost due to Arctic warming will continue to release nutrients and organic carbon into these systems and increase ponding in some regions, likely stimulating higher water concentrations of MeHg. Greater hydrological connectivity from permafrost thawing may potentially increase transport of MeHg from thaw ponds to neighbouring aquatic ecosystems.